We describe a paper-based microfluidic device for iron(III) assay in an applied instrumental analysis laboratory. Simple and convenient devices were fabricated by the students using patterning of filter paper to investigate three different procedures: drawing a boundary with eyeliner or wax pen; drawing a boundary with waterproof permanent marker pen; or drawing no boundary. These test papers were used for the analysis of iron(III) in water samples by smartphone-based colorimetric detection with thiocyanate. Additionally, the interference effect of other metal cations was done to further demonstrate practical utility. The results for field sample detection using this microdevice were satisfactory and show promise for use in other applications.